It is known to provide an electronic still video camera; that is, apparatus which electrically stores signals representing a still image. The stored image may then be read and displayed, for example, on a conventional television or monitor. Conveniently, therefore, the signals are provided in a suitable television image format; the stored signals may, for example, comprise luminance and color difference signals modulated on to a high frequency carrier and recorded on a magnetic or magneto-optical record carrier such as disk.
One example of such a camera is shown in U.S. Pat. No. 4,956,715, granted to the same assignee as the present invention.
In a conventional (non-electronic) camera, the silver halide film has a wide dynamic range; in other words, if the exposure provided by the shutter and iris settings is too low, it is generally possible to retrieve an acceptable image by increasing the development time. However, by comparison, the dynamic range of an electronic camera tends to be much smaller so that if the iris and shutter settings are too large or too small during exposure of the electronic image sensor which provides the electronic video signal, the image formed will be entirely overexposed or entirely underexposed (all white or all black), or almost so.
Manipulating the gain of the electronic signal produced by the sensor (equivalent to controlling the development time of the conventional film) can restore the average level of the electronic signal, but cannot restore the image contrast which has been lost due to the incorrect exposure. It is therefore important in electronic cameras to provide the correct exposure for the image sensor.
With moving image video cameras, it is known to provide a feedback loop to continuously control the iris setting in dependence upon previous sensor outputs. However, this is not possible with a still video camera because the camera is not operative before a given video picture is taken, so continuous control is not possible.
Accordingly, the present assignee has developed a technique for exposure control for an electronic still video camera. As described in U.S. Pat. No. 4,956,715, a photosensitive element is provided which has a relatively wide dynamic range, compared to the image sensor, which receives a light level corresponding to that receivable by the image sensor and provides an electrical output signal indicating the light level. This output is used to control the iris setting and/or the shutter speed (it may, for example, be desirable to allow the user to control the iris and accordingly to control only the shutter speed, or vice versa) to provide an approximate exposure setting.
However, the approximate exposure setting is not sufficiently accurate to be directly usable, due to the different responses to the light of the photometric sensor and the image sensor. Accordingly, a second exposure control process is also performed in which, when a picture is to be taken, the image sensor is first exposed to light and its output over the image area is measured. The output is then employed to set the shutter and/or iris, and the picture is taken by re-exposing the image sensor to light for the thus-derived exposure period, and storing the image sensor output (in a suitable format).
A further problem which occurs with cameras of all kinds for taking color pictures is that of the color of illuminating light. Illumination sources such as sunlight, light bulbs, or neon sources produce light colors and color distributions which vary widely. Many light sources act approximately like black body radiators, and it is therefore conventional to describe their color and distribution in terms of a color temperature. The human eye is extremely tolerant to variations in color temperature, and will tend to treat a white object as white whatever the illuminating color. However, the human eye quickly recognizes a color cast in a picture.
Accordingly, it is very desirable in a color camera to provide a color balancing circuit, which is arranged to derive an indication of the color of the illumination and to process the picture signal to compensate accordingly. For example, if the illumination were read, the color video signal would be processed to reduce the amount of red present therein, to restore the color balance. Since information on the illumination is not generally available to the camera, the illumination color is inferred by deriving the average color of the scene.
In a moving picture video camera, it is possible to keep a moving temporal average of previous output of the image sensor (over a window of about 2-10 seconds) to provide an approximate indication of the average scene color and hence color temperature. However, with a still camera this is not possible because as noted above the camera is generally not operative except when a picture is being taken.
One method employed in the prior art is to provide a separate color temperature detecting circuit, which comprises typically three photosensors each disposed under a respective color filter (for example a red filter, a green filter and a blue filter), positioned behind an optical diffuser (for example a frosted glass element) so that they receive light from substantially the whole scene. The outputs of the three sensors indicate the relative magnitudes of three different colors in the scene, and the amount by which they deviate from a predetermined ratio is employed to correct the color balance of the output of the image sensor. Typically, the image sensor provides a separated color or color difference video signals, and the amplitude of these signals is controlled by a controllable gain element in dependence upon the color temperature measuring sensor outputs.
However, in practice the color balance control provided by this method is found not to be ideal. Firstly, the color filters provided in the color sensor may not correspond exactly to those provided on the image sensor. Secondly, the color response characteristics of the image sensor are typically very different to those of the color measuring sensor. Thirdly, providing a separate sensor is expensive.
Prior proposals for eliminating the separate sensor are known. Such proposals instead use the output of the image sensor to derive a measure of the color balance. However, there are some problems with such proposals. One problem is that, whereas the color measuring sensor in the prior art was arranged to receive a wide view through a diffuser plate, the image sensor receives a view through the focussing lenses of the camera and may therefore receive a view which corresponds to a close up of a particular object. If this object is substantially monochrome, for example, all red or all blue, the output of the image sensor will correspondingly be predominantly monochrome and there is a danger that this will be assumed to correspond to colored light rather than a colored object, leading to an opposite color balance correction which would (incorrectly) render the object which and all other objects incorrect. In a video camera, this problem is overcome by integrating or averaging the output of the photosensor over a relatively long period of time (seconds or minutes). Since the camera will not usually point at a single object for a very long period of time, the color balancing information is derived corresponding to a number of objects of picture areas and correspondingly does not depend upon a monochrome object. However, this is not possible with a still camera for obvious reasons.
EP-A-0207543 describes an electronic video camera including a frame transfer type image sensor and having a white balance control circuit which is controlled from the image sensor output during a frame blanking period in which the light to the sensor is passed through a light diffuser comprising a plate of frosted glass inserted temporarily in front of the sensor. This broadens the field of view of the sensor and reduces the dependence on monochrome subjects. Adjustment of the white balance occurs between successive picture taking intervals of a moving picture sequence.
However, insertion and removal of the diffuser element is time consuming and involves intricate and expensive electro mechanical actuators. In addition, it can reduce the time for forming the pre-exposed image, which is already relatively short compared with the time for forming the image itself.
EP-A-0394018 (invented by the present inventor and assigned to the present applicant), forming part of the state of the art by virtue of Art 54(3) only, describes, in a first embodiment (FIG. 1) a camera in which a frame of an image is captured by the image sensor, and the image sensor output is stored in an image memory and also used to calculate color balance direction data. The image information stored in the memory is then read out and corrected in accordance with the color balance correction data. The color balance correction is calculated by averaging color information over the captured frame and consequently the image memory is necessary to provide a one frame delay while the color balance correction data is being calculated. In some applications, however, the expense of a further image memory (which must, of course, have a substantial size), together with its associated analogue-to-digital and digital-to-analogue conversion means, is not acceptable. Further, such a device increases the power usage (and reduces the battery lifetime) of the camera, and increases the size of the electronic circuit.
In a second embodiment (FIG. 5) of EP-A-0394018, the memory is omitted. The color balance data is instead derived by initially exposing the image sensor to the scene; optically obtaining wide-scene information by de-focussing, zooming or using a frosted defocussing plate so as to reduce the depence on monochrome objects; deriving color representative signals and then integrating the signals over the frame to provide color balance control signals.